Melting or softening of small particles under controlled conditions to convert them to generally ellipsoidal form is known. "Atomization," "fire polishing" and "direct fusion" techniques have been used.
Atomization methods involve first melting myriad raw material particles together to convert them to molten, i.e. bulk liquid, glass. Such bulk liquid typically contains far more than hundreds or thousands of times the amount of raw material required to make a single product particle. A thin stream of this molten glass is "atomized" by dropping it into a disruptive air jet, subdividing the stream into fine, molten droplets. The droplets are kept away from one another and from other objects until they have been cooled and solidified. Then they can be recovered as substantially discrete, generally ellipsoidal glass particles.
Describing atomization of glasses, Katz and Milewski, at page 303 of their "Handbook of Fillers and Reinforcements for Plastics," Van Nostrand Reinhold Company, New York, N.Y., 1978, explain that a glass batch, which initially includes crystalline materials, may contain sand, soda ash, dolomite, feldspar and other ingredients. When melted and thoroughly mixed so that the ingredients are no longer crystalline, the resultant bulk liquid material is then atomized. Glassy, amorphous, generally ellipsoidal particles are formed.
In fire-polishing, discrete solid particles of material having irregular shapes are heated to the softening or melting temperature of the material while suspended and dispersed in a hot gaseous medium. As particles become soft or molten, surface tension forms them into ellipsoidal shapes. If kept in suspension until cooled below the temperatures at which they "freeze" and become solid, the particles may then be recovered as generally discrete glassy ellipsoids.
Particulate feed materials for fire-polishing may be in the form of amorphous crushed glass solids when initially introduced into the gaseous medium. Thus, at page 302 of Katz and Milewski it is shown that particles of crushed and screened glass, such as plate glass, various glass cullets and bottle glass, all amorphous materials, may be suspended and dispersed in a hot gaseous medium and softened or melted to form them into ellipsoidal shapes.
Direct fusion bears some resemblance to fire-polishing. Feed particles with irregular shapes, including individual solid particles and/or adherent groups of such particles that are sometimes referred to as "clusters" or "agglomerates," are heated and softened or melted while in suspension and dispersion in a hot gaseous medium to form them into molten, generally ellipsoidal shapes, followed by cooling, freezing and recovery. Direct fusion draws its name in part from the fact that its feed particles directly undergo conversion to glassy or amorphous form in the ellipsoid-forming step, without prior conversion to bulk liquid form.
It is believed that a group of several mutually adherent feed particles, whether they become adherent prior to or during the ellipsoid-forming step, can melt and fuse to form a single, generally ellipsoidal particle of proportionately larger diameter. Thus, when these fused products are produced by direct fusion, whether they are formed from such groups of feed particles and/or from particles that remain discrete during fusion, the resulting generally ellipsoidal particles generally exhibit the varying average chemical compositions of the particles and/or groups of particles from which the ellipsoids are respectively formed, except that there may be relatively small losses of ingredients through high-temperature volatilization. Direct fusion products do not necessarily have the more uniformly similar particle-to-particle composition expected of particles produced from bulk liquid glass.
Atomization and fire polishing of glasses may be described as indirect methods. Their feed materials have been formulated from glass-making raw materials which were melted and homogenized in the form of bulk liquid prior to entering the ellipsoid-forming step. Consequently, in indirect methods, the individual chemical identities of the glass-making raw materials have been merged into an average composition which is uniformly present in the respective ellipsoids so produced.
Illustrations of direct fusion may be found in Japanese published patent applications HEI 2[1990] 59416 and HEI 2[1990] 199013, published respectively on Feb. 28, 1990 and Aug. 7, 1990. Therein, Morishita, et al and Shimada, et al respectively suggest fusing high purity silica particles with sizes measured in microns. The resultant products are for example useful as fillers in plastics.
Also, Klingaman and Ehrenreich, in U.S. Pat. Nos. 4,268,320 and 4,294,750, teach how to recover pyroplastoids, fused, substantially non-hollow alumino-silicate glassy ellipsoidal particles from fly ash found in the flue gases of coal fired boilers. These fused particles are also used as fillers in plastics, and for other purposes.
Ellipsoidal particles recovered from fly ash are generally economical, but can suffer from the disadvantage of containing colorants that are expensive if not virtually impossible to remove. Such colorants render these ellipsoids undesirable for certain end use applications.
Atomization processes can produce products comparatively free of undesirable colorants. However, these do not readily produce abundant quantities of some of the smaller particle sizes that are desired, for example particles smaller than 25 microns in average size.
Fire polishing of crushed or ground commercial glasses can be used to make very small particles having low color levels. However, the high cost of milling these amorphous materials to small sizes has contributed to the high cost of making small, uncolored particles by this route.
Direct fusion processes heretofore disclosed for converting crystalline silica to amorphous ellipsoids appear capable of producing white or transparent particles in very small sizes, but tend to be quite expensive due to the energy required to fuse these high-melting materials. It has been suggested that these processes be applied to broad categories of mineral materials, including alumino-silicates, metal silicates and other inorganic powders. However, whether this suggestion is practical, which of the myriad types and forms of raw materials available in these categories should be employed, and how this suggestion should be implemented to overcome the above difficulties, have yet to be made clear.
Thus, it is believed that a need remains for improvements in ellipsoidal fused particulate products, and in methods for producing them. The present invention seeks to fulfill this need.